1. Field of the Invention
Embodiments of the present invention generally relate to a method of fabricating thin film transistors (TFTs).
2. Description of the Related Art
Current interest in TFT arrays is particularly high because these devices may be used in liquid crystal active matrix displays (LCDs) of the kind often employed for computer and television flat panels. The LCDs may also contain light emitting diodes (LEDs) for back lighting. Further, organic light emitting diodes (OLEDs) have been used for active matrix displays, and these OLEDs require TFTs for addressing the activity of the displays.
TFTs made with amorphous silicon have become the key components of the flat panel display industry. Unfortunately amorphous silicon does have its limitations such as low mobility. The mobility required for OLEDs is at least 10 times higher than that achievable with amorphous silicon. In addition, OLED display is more sensitive to the Vth shift since it is a current driven device. Vth shift of amorphous TFTs under either high current or high bias voltage is an issue to be addressed. Polysilicon, on the other hand, has a higher mobility than amorphous silicon. Polysilicon is crystalline, which leads to poor local non-uniformity. Due to the requirement of a complex annealing process to make the polysilicon film, it is more difficult and/or more costly to make large area displays using polysilicon. Due to the limitations of amorphous silicon, OLED advancement has been difficult.
In recent years, transparent TFTs have been created in which zinc oxide has been used as the active channel layer. Zinc oxide is a compound semiconductor that can be grown as a crystalline material at relatively low deposition temperatures on various substrates such as glass and plastic. Zinc oxide based semiconductors can be made as amorphous material through doping. Therefore, doped zinc oxide will avoid the non-uniformity issues that may result from a non-uniform grain structure. Amorphous semiconductors such as zinc oxide are more easily implemented in current display manufacturing processes using bottom gate TFT structures.
Therefore, there is a need in the art for TFTs having transparent active channels with high mobility.